My research is in general about interactions between atoms and between atoms and light. Using lasers-cooling techniques one can chill dilute clouds of atoms down to the micro-Kelvin regime (i.e. down to 0.0000001 degrees above absolute zero temperature, or -273.15 Celsius). For this feat Bill Phillips, Steven Chu and Claude Cohen-Tannoudji received the Nobel Prize in 1997. Using evaporation one can get down another factor 1000 or so.

There are many reasons why one wants to chill atoms to these temperatures. One is that the atoms can undergo a phase transition into a state with quantum-mechanical properties, a Bose-Einstein Condensate. This state was achieved for the first time in 1995 (Nobel Prize in 2001 to Wolfgang Ketterle, Eric Cornell and Carl Weiman). Since then there has been a tremendous progress in this field, including Bose-Einstein condensation of molecules, Cooper pairing of fermionic atoms, and a new states of matter of ultracold atoms trapped in periodic light potentials.

Another reason to work with ultracold atoms is that they are (relatively) easy to manipulate in various ways. For instance, using a magnetic field one can tune the interaction between two atoms to essentially any value. Atoms can be kicked, bounced or trapped using lasers. At these temperatures the atoms move with only a few cm per second. This makes them ideal for extremely accurate measurements, which can be used to test very fundamental laws of nature. Using very clever laser techniques (which gave Ted Hänsch and Jan Hall their share of the Nobel Prize in 2005) the 1s-2s line in hydrogen has been measured with an accuracy of 2 parts in 10^14. Probably, the most accurate measurement of anything, anytime.

Some of my current research interests are described below.



An antihydrogen event at ATHENA

The cylindrical detectors surrounding the experiment see the particles coming from annihilation of an electron-positron pair (red lines) and an antiproton (red lines). The tracks are traced back to a common point where the antihydrogen annihilated.

For every particle there is an antiparticle, which is (as far as we know) a perfect mirror image, except that it has opposite charge. Mixing antiprotons and antielectrons (called positrons) one can combine them to antihydrogen, i.e. the antimatter counterpart to ordinary hydrogen. This was done by the ATHENA experiment at CERN in 2002.

I work together with the ALPHA experiment, which is a continuation of ALPHA. The goal of this experiment is to trap antihydrogen atoms, and to study them using laser spectroscopy. In this way one may find some tiny difference between matter and antimatter. Possibly, such a difference could explain why our present universe seems to contain only ordinary matter, although at the Big Bang we would expect matter and antimatter to have been created at similar amounts.

Trapping of antimatter is difficult since it annihilates as soon as it meets ordinary matter. For charged particles, such as the antiprotons and positrons electric fields can be used to hold them in place. But the antihydrogen atom is neutral so this is not possible. Instead one has to use magnetic forces. These are very weak, so the antihydrogen has to be very cold, less than about a Kelvin, to stay in the trap. Creating antihydrogen at temperatures low enough to allow trapping is a major challenge.

My own work has centered on two areas:

  • Formation of antihydrogen. Antihydrogen is formed when antiprotons are injected into a positron plasma. The reaction involves two positrons, since one extra is required to take away the energy released when the bound state is formed. The antihydrogen is initially formed in a very loosely bound and fragile state. More collisions is required to achieve a more tightly bound antiatom which can survive through trapping. But the collisions can also destroy the antiatom. The process is further complicated by the electric and magnetic fields present in the trap.
  • Interaction of antihydrogen with ordinary matter. I have calculated cross sections for scattering of antihydrogen on ordinary hydrogen or Helium at low energies. These were the first quantum mechanical calculations of these systems. Possible reactions include elastic scattering (the atom and antiatom simply bounces off again), annihilation, formation of positronium (the bound state of an electron and a positron), formation of protonium (proton and antiproton), and even a rather exotic "molecule" consisting of a hydrogen atom and an antihydrogen atom.

The ALPHA trap

The coils create a spatially dependent magnetic field. At the minimum of the magnetic field antihydrogen can be trapped if it is cold enough to not escape.

Brownian Motors and Ratchets

Ratchets are systems which create directed motion of particles (e.g. atoms) from periodic potentials giving zero average force. A Brownian Motor is a ratchet where random motion (usually thermal fluctuations) are important for the driving mechanism. This may see to violate the second law of thermodynamics, which we obviously cannot allow. The trick is that the systems are driven out of thermal equilibrium. This kind of motors can be very important for transport at the micro-scale (where ordinary motors based on thermal gradients are hard to realize).

The various mechanisms which give ratchet effects are quite general. One physical system where several different ratchet schemes has been realized is cold atoms in periodic light potentials (optical lattices). Together with Anders Kastberg's group in Umeå, I have investigated a quite interesting such realization, built on two phase-shifted symmetric potentials, and operational in all three dimensions.

Currently I am working together with Paul Halkyard, on ratchet mechanisms in the quantum domain (also using cold atoms). In fact it is not completely trivial to give a precise definition of a ratchet in the quantum domain. We are just now looking at how ratchet effects may occur at special resonant parameters, and how this is related to the symmetries of the system.

Few-body Physics

Efimov states are rather peculiar bound states of three particles, predicted by Vitaly Efimov in 1970. An Efimov state may exist even though two atoms cannot bind together. They also have remarkable scaling properties, completely independent of which kind of particles the Efimov state consists of. If all lengths are re-scaled by a factor 22.7 (a number which can be derived analytically), and energies by 22.7^2=585, then another Efimov state appears. I find this kind universality of the three-body interaction very fascinating. Recently (2006) Rudi Grimm and his group in Innsbruck showed that Efimov states are not just a theorists fiction, when they detected the first such state in ultracold Caesium. Since then, several other groups have followed suite.